We have to assume some handwaving but I think these would be the key for Canada (as we know it) to have a much higher population.
Control of the St. Lawrence River and Great Lakes (at the very least some portion of America on the south shore of lake Superior) would be an absolute must for a truly massive population. A much earlier St. Lawrence Seaway (easily doable with 1830's technology) would cause a huge population boom in Upper Canada and make it far more economically viable due to cheaper imports/exports. Secondly, you could start the transcontinental railroad a decade sooner and it would be vastly cheaper and get built faster if it didn't have to slog through northern Ontario. This means you could start settling the prairie a decade or so earlier which would have long reaching impacts on the Canadian population.
I still think 300 million is nigh impossible but this Canada could probably have 100 million without too many changes (assuming a long period of relative peace and friendly relations with the United States).
Control of the St. Lawrence River and Great Lakes (at the very least some portion of America on the south shore of lake Superior) would be an absolute must for a truly massive population. A much earlier St. Lawrence Seaway (easily doable with 1830's technology) would cause a huge population boom in Upper Canada and make it far more economically viable due to cheaper imports/exports. Secondly, you could start the transcontinental railroad a decade sooner and it would be vastly cheaper and get built faster if it didn't have to slog through northern Ontario. This means you could start settling the prairie a decade or so earlier which would have long reaching impacts on the Canadian population.
I still think 300 million is nigh impossible but this Canada could probably have 100 million without too many changes (assuming a long period of relative peace and friendly relations with the United States).